Data from: Evolution to environmental contamination ablates the circadian clock of an aquatic sentinel species

Environmental contamination is a common cause of rapid evolution. Recent work has shown that Daphnia pulex, an important freshwater species, can rapidly evolve increased tolerance to a common contaminant, sodium chloride (NaCl) road salt. While such rapid evolution can benefit organisms, allowing them to adapt to new environmental conditions, it can also be associated with unforeseen tradeoffs. Given that exposure to environmental contaminants can cause circadian disruption, we investigated whether the circadian clock was affected by evolving a tolerance to high levels of road salt. By tracking the oscillations of a putative clock gene, period, we demonstrated that D. pulex express per mRNA with approximately 20-hour oscillations under control conditions. This putative circadian rhythm was ablated in response to high levels of salinity; populations adapted to high NaCl concentrations exhibited an ablation of period oscillation. Moreover, we showed that while gene expression is increased in several other genes, including clock, actin, and Na+/K+-ATPase, upon the adaptation to high levels of salinity, per expression is unique among the genes we tracked in that it is the only gene repressed in response to salt adaptation. These results suggest that rapid evolution of salt tolerance occurs with the tradeoff of suppressed circadian function. The resultant circadian disruption may have profound consequences to individuals, populations, and aquatic food webs by affecting species interactions. In addition, our research suggests that circadian clocks may also be disrupted by the adaptation to other environmental contaminants.

环境污染是引发快速演化的常见诱因。近期研究表明，作为重要淡水物种的大型溞（Daphnia pulex）能够快速演化出对常见污染物——氯化钠（NaCl，道路融雪盐）的耐受能力。尽管此类快速演化可使生物获益，帮助其适应全新环境条件，但也可能伴随难以预见的适应性权衡。鉴于环境污染物暴露可引发昼夜节律紊乱（circadian disruption），本研究探究了演化获得高浓度道路融雪盐耐受能力的过程是否会影响生物的昼夜节律钟（circadian clock）。通过追踪推定节律基因period（per）的表达振荡情况，本研究证实：在对照条件下，大型溞的per mRNA表达呈现约20小时的振荡周期。该推定的昼夜节律在高盐度胁迫下被消除；适应高浓度NaCl的种群则表现出period基因振荡的消失。此外，本研究发现：在适应高盐环境的过程中，包括时钟基因（clock）、肌动蛋白（actin）以及钠钾腺苷三磷酸酶（Na+/K+-ATPase）在内的多个其他基因的表达均会上调，而per基因的表达是本研究追踪的所有基因中唯一被盐适应过程抑制的基因。上述结果表明，盐耐受能力的快速演化会伴随昼夜节律功能受抑制的适应性权衡。由此产生的昼夜节律紊乱可能通过影响物种间相互作用，对个体、种群以及水生食物网造成深远影响。此外，本研究提示，对其他环境污染物的适应过程同样可能导致昼夜节律钟的紊乱。